Buggy code:
public abstract class FormComponent < T > extends LabeledWebMarkupContainer implements IFormModelUpdateListener , IFormVisitorParticipant { private static final Logger logger = LoggerFactory . getLogger ( FormComponent . class ) ; private class MessageSource implements IErrorMessageSource { private final Set < String > triedKeys = new LinkedHashSet < String > ( ) ; public String getMessage ( String key ) { } private String prefix ( String prefix , String key ) { } private String getString ( Localizer localizer , String key , Component component ) { } public String substitute ( String string , Map < String , Object > vars ) throws IllegalStateException { } private Map < String , Object > addDefaultVars ( Map < String , Object > params ) { } private String getLabel ( ) { } } private class ValidatableAdapter implements IValidatable < T > { public void error ( IValidationError error ) { FormComponent . this . error ( error ) ; } public T getValue ( ) { } public boolean isValid ( ) { } } public static String VALUE_SEPARATOR = ";" ; private static final String [ ] EMPTY_STRING_ARRAY = new String [ ] { "" } ; private static final short FLAG_REQUIRED = FLAG_RESERVED3 ; private static final String NO_RAW_INPUT = "[-NO-RAW-INPUT-]" ; private static final long serialVersionUID = 1L ; protected static final short FLAG_CONVERT_EMPTY_INPUT_STRING_TO_NULL = FLAG_RESERVED1 ; public static final < R > R visitFormComponentsPostOrder ( Component component , final IVisitor < ? extends FormComponent , R > visitor ) { } public static final < R > R visitComponentsPostOrder ( Component component , final IVisitor < Component , R > visitor ) { } private transient T convertedInput ; private String rawInput = FormComponent . NO_RAW_INPUT ; private String typeName ; private Object validators = null ; public FormComponent ( final String id ) { } public FormComponent ( final String id , IModel < T > model ) { } public final FormComponent < T > add ( final IValidator < ? super T > validator ) { } public final FormComponent < T > add ( final IValidator < ? super T > ... validators ) { } public boolean checkRequired ( ) { } public final void clearInput ( ) { } public void error ( IValidationError error ) { if ( error == null ) { throw new IllegalArgumentException ( "Argument<seq2seq4repair_space>[[error]]<seq2seq4repair_space>cannot<seq2seq4repair_space>be<seq2seq4repair_space>null" ) ; } FormComponent < T > . MessageSource source = new MessageSource ( ) ; String message = error . getErrorMessage ( source ) ; if ( message == null ) { <START_BUG> StringBuffer buffer = new StringBuffer ( ) ; <END_BUG> buffer . append ( "Could<seq2seq4repair_space>not<seq2seq4repair_space>locate<seq2seq4repair_space>error<seq2seq4repair_space>message<seq2seq4repair_space>for<seq2seq4repair_space>component:<seq2seq4repair_space>" ) ; buffer . append ( Classes . simpleName ( getClass ( ) ) ) ; buffer . append ( "@" ) ; buffer . append ( FormComponent . getPageRelativePath ( ) ) ; buffer . append ( "<seq2seq4repair_space>and<seq2seq4repair_space>error:<seq2seq4repair_space>" ) ; buffer . append ( error . toString ( ) ) ; buffer . append ( ".<seq2seq4repair_space>Tried<seq2seq4repair_space>keys:<seq2seq4repair_space>" ) ; Iterator < String > keys = source . triedKeys . iterator ( ) ; while ( keys . hasNext ( ) ) { buffer . append ( keys . next ( ) ) ; if ( keys . hasNext ( ) ) { buffer . append ( ",<seq2seq4repair_space>" ) ; } } buffer . append ( "." ) ; message = buffer . toString ( ) ; FormComponent . logger . warn ( message ) ; } error ( new ValidationErrorFeedback ( error , message ) ) ; } public final T getConvertedInput ( ) { } public final void setConvertedInput ( T convertedInput ) { } public Form < ? > getForm ( ) { } public String getInput ( ) { } public String [ ] getInputAsArray ( ) { } public String getInputName ( ) { } public final String getRawInput ( ) { } @ SuppressWarnings ( "unchecked" ) public final Class < T > getType ( ) { } public String getValidatorKeyPrefix ( ) { } public final List < IValidator < ? super T > > getValidators ( ) { } public final String getValue ( ) { } public final boolean hasRawInput ( ) { } public final void inputChanged ( ) { } public final void invalid ( ) { } public boolean isInputNullable ( ) { } public boolean isMultiPart ( ) { } public boolean isRequired ( ) { } public final boolean isValid ( ) { } public boolean processChildren ( ) { } public final void processInput ( ) { } public FormComponent < T > setLabel ( IModel < String > labelModel ) { } public void setModelValue ( final String [ ] value ) { } public final FormComponent < T > setRequired ( final boolean required ) { } public final FormComponent < T > setType ( Class < ? > type ) { } public void updateModel ( ) { } public final void valid ( ) { } public void validate ( ) { } @ SuppressWarnings ( "unchecked" ) private void validators_add ( final IValidator < ? super T > validator ) { } @ SuppressWarnings ( "unchecked" ) private IValidator < T > validators_get ( int index ) { } private int validators_size ( ) { } @ SuppressWarnings ( "unchecked" ) protected void convertInput ( ) { } private void reportValidationError ( ConversionException e , ValidationError error ) { } @ SuppressWarnings ( "unchecked" ) protected T convertValue ( String [ ] value ) throws ConversionException { } protected String getModelValue ( ) { } protected final int inputAsInt ( ) { } protected final int inputAsInt ( final int defaultValue ) { } protected final int [ ] inputAsIntArray ( ) { } @ Override protected void internalOnModelChanged ( ) { } @ Override protected void onComponentTag ( final ComponentTag tag ) { } @ Override protected void onDetach ( ) { }

Fix:
StringBuilder buffer = new StringBuilder ( ) ;